Flow cytometric analysis of red blood cells is used to monitor levels of Hemoglobin F (HbF) and Hemoglobin F-containing reticulocytes (F-cells) in sickle cell disease (SCD) patients and to assess their responses to different treatments. [unreadable] Lately, erythropoietin is being used more widely in the management of sickle cell disease, often in conjunction with hydroxyurea (HU). We have followed for a median of 11 months patients with sickle syndromes that were treated with erythropoietin or darbepoietin (both referred to as EPO). The median age of sickle syndrome patients was 51 (24 to 70) years; most patients had sickle-associated pulmonary hypertension. There was three study groups: (1) patients with pulmonary hypertension that were given EPO for reticulocytopenia on HU, (2) patients with pulmonary hypertension and renal insufficiency that were given EPO and HU concurrently, and (3) patients treated with EPO for miscellaneous reasons. [unreadable] Results show that the percent of HbF and F-cells rose in all evaluable patients. Results suggest that EPO therapy may allow more aggressive HU dosing in high-risk SCD patients and in the setting of mild renal insufficiency, common to the aging sickle cell population. EPO appears to be safe in SCD, particularly when used in conjunction with HU. One paper is published.